AM NPP - High temperature solution annealing of AM 316L

The effect of different solution annealing (SA) treatments on the material properties of L-PBF produced 316L specimens was investigated. Hot Isostatic Pressing (HIP) is often the heat treatment of choice for L-PBF 316L as it results in good mechanical properties and isotropic microstructure, whereas the standard solution annealing cycle at 1066°C is not sufficient at homogenizing the material structure and properties. High temperature annealing was considered as an alternative for the more expensive HIP process, and according to published research, the higher annealing temperatures produce properties more close to conventionally manufactured alloy.

Cylindrical and rectangular bars were printed using L-PBF and the three different thermal post processes were applied: 1066°C/1h, 11150°C/1h and 1200°C/1h. All parts were heat treated in a vacuum furnace and stress relieved (650°C/2h) prior to solution annealing. The 1066°C annealing complies with the current 316L AM standard. After machining, characterization of mechanical properties was done by static tensile tests and Charpy-V impact tests. Samples were prepared for microstructure (SEM/EBSD) and chemical analysis.

The printed test specimen had very low porosity and a chemical composition comparable to the feedstock powder with no excess oxidation. The microstructure evolved from partially recrystallized and anisotropic at 1066°C to nearly fully recrystallized at 1200°C. The tensile properties decreased with increasing annealing temperature and fulfilled the minimum requirements specified in AM standard (Re ≥ 205 MPa, Rm ≥ 515 MPa, A ≥ 30 %) in all conditions. The impact energies followed the same trend and for the 1200°C condition the average impact energy was below 40 J, which is the minimum requirement in standards SFS-EN 13480, SFS-EN 13445 that are relevant for materials used in nuclear applications.